1. Technical Field
The present invention relates to a progressive power lens for correction of presbyopia and to a method of designing the same.
2. Related Art
Among technical progress of spectacle lenses, lens designing includes a technical field of designing called as custom designing or individual designing.
In this field of designing, spectacle lenses, in particular, progressive power lenses used for correcting presbyopia, are designed to optimize their optical characteristics and optical performance, in consideration not only of opthalmologic prescriptions for users but also of lifestyle different from one individual to another (e.g., intended use of spectacles, scenes, use frequency) and lens-wearing conditions such as positioning of lenses assembled into and worn as spectacles with respect to eyes and orientations and inclinations of the lenses with respect to the line of sight.
According to known methods of manufacturing lenses by custom designing, spectacle lenses are designed with use of customization information related to users' eyes per se and customization information related to users' lifestyles and the like (Document 1: Japanese Patent No. 3944914, Document 2: JP-T-2003-521741, Document 3: WO2005/092173).
For instance, Document 1 discloses a method of manufacturing lenses by custom designing with use of customization information related to users' eyes and customization information related to users' lifestyles. The customization information related to users' eyes contains opthalmologic prescription information such as information about power (spherical power, cylindrical power and cylinder axis), added power and convergence of eyes in a region from a distance portion to a near portion, as well as fitting data for wearing spectacles such as vertex distances, pantoscopic angles and bend angles of frames. According to Document 1, basic designing parameters such as wideness and positions of far point and near point are customized in accordance with the information related to lifestyles, and the progressive surface is computed based on the customized parameters. Further, the optical characteristics are corrected in accordance with the fitting information such as the pantoscopic angles.
In the above hitherto-manufactured spectacle lenses, the pantoscopic angle is defined as a standard setting angle.
FIGS. 1A and 1B show a typical structure of a progressive power lens. FIG. 1A is a front view showing the progressive power lens 1, and FIG. 1B shows a vertical cross-section thereof (hatching is omitted).
In FIGS. 1A and 1B, the progressive power lens 1 includes two refractive surfaces: a front surface 2 (hereinafter referred to as outer surface) and a back surface 3 (hereinafter referred to as inner surface). The progressive power lens 1 also includes: a distance portion 4 for distant vision; a near portion 5 for near vision; and an intermediate corridor 6a of which refractive power is progressively changed from the distance portion 4 to the near portion 5.
The refractive power of the near portion 5 is greater than that of the distance portion 4 by the addition power. A main fixation line 7, which is a position on the lens through which the line of sight frequently passes when the lens is worn, is substantially vertically formed in substantially the middle of the lens. In the near portion 5, the main fixation line 7 comes close to the nose side (right side in FIG. 1A) due to convergence for near-range vision. A region between the distance portion 4 and the near portion 5 is called as an intermediate portion. This intermediate portion includes: an intermediate corridor 6a located in the vicinity of the main fixation line 7, aberration of which is relatively small; and intermediate lateral parts 6b located externally to the intermediate corridor 6a. The intermediate lateral parts 6b, in which astigmatic aberration and distortion aberration are increased, is a region not suitable for clearly viewing an object without distortion. A fitting point 9, through which the line of sight passes at the primary position (position in a horizontal front view) when the lens is mounted in a spectacle frame, is located on the main fixation line 7, and a prism reference point 8 is located at a position vertically below the fitting point 9 by several millimeters. The prism reference point 8 is frequently set to be located at the center position of the vertical width of a spectacle frame (not shown) when the lens is mounted into the spectacle frame.
The above-described distance portion 4, near portion 5 and intermediate portions 6a and 6b of the progressive power lens are provided on a single surface called as a progressive surface. The progressive surface is provided on the outer surface or on the inner surface. In some lenses, the progressive surface is provided on both of the outer and inner surfaces.
FIGS. 2A and 2B are for explaining a pantoscopic angle and directional components of a refractive power on the main fixation line. FIG. 2A is a front view showing the progressive power lens, and FIG. 2B shows a vertical cross-section passing through the fitting point (hatching is omitted).
In FIGS. 2A and 2B, the prism reference point 8 is provided at the geometric center of the lens. When the lens is mounted into a spectacle frame, this geometric center, i.e., the prism reference point 8, is coincident with the center of the lens shape. As shown in FIG. 2B, in the above-configured lens, an angle formed between a normal line normal to the outer surface at the prism reference point 8 in the cross sectional view and a horizontal plane passing through the prism reference point 8 is defined as a pantoscopic angle PA. While the pantoscopic angle PA may be defined in various ways, the pantoscopic angle herein is defined as an inclination of the normal line normal to the outer surface (front surface) at the center position of the mounted lens with respect to the visual axis at the primary position. Hereinafter in this description, the prism reference point of the outer surface serves as the origin, a Z axis (+) is set in a direction extending toward the eye in the horizontal plane, and a Y axis (+) is set in a direction extending upward in a vertical plane orthogonal to the Z axis. As in FIG. 2B, the pantoscopic angle PA takes positive values (+) when formed in a direction in which the distance portion is away from the eye and the near portion is close to the eye, i.e., in a direction in which the lens is laid down.
In FIG. 2A, a refractive power along the main fixation line 7 is a refractive power within a cross-section 10, and a refractive power in a direction orthogonal to the main fixation line 7 is a refractive power within a cross-section 11.
Lens manufacturers typically design lenses by setting the pantoscopic angle PA, i.e., a design parameter, at a predetermined value within a range of 8 to 12 degrees, and the predetermined value serves as the standard value. Then, lens manufacturers communicate this information to spectacle retailers, and the spectacle retailers perform fitting adjustment on a user's selecting spectacle frame so that the spectacle frame is tilted at the standard pantoscopic angle.
Typically in such hitherto-manufactured progressive power lenses, the pantoscopic angle PA is set at a predetermined value, i.e., a standard value. However, some spectacle frames are difficult to adjust due to their designs and materials, so that the pantoscopic angle may be set at an angle greatly deviated from the pantoscopic angle initially targeted in the designing. Such deviation will result in improper optical performance that is deviated from the originally-targeted optical performance. The reasons therefor will be described in detail below with reference to embodiments.
A progressive power lens of which distance portion is prescribed for myopia use will be described as Conventional embodiment 1. The progressive power lens of Conventional embodiment 1 has: a spherical power SPH of −4.00 diopter; a cylindrical power CYL of 0.00 diopter; an addition power ADD of 2.00 diopter; a corridor length PL of 14 mm; a refractive index N of 1.67; a pantoscopic angle PA of 10 degrees; and a base curve BC of 3.00 diopter. The pantoscopic angle PA is defined as positive when formed in the direction in which the lens is laid down. The base curve BC is a surface power of the outer surface of the distance portion of the progressive power lens, and the corridor length PL is a vertical length of the intermediate corridor 6a. 
FIG. 3 is for explaining the optical characteristics on the main fixation line when the progressive power lens of Conventional embodiment 1 is worn at different pantoscopic angles. The graphs (A) to (D) in the upper row illustrate the results when the pantoscopic angle PA=0 degree, the graphs (E) to (H) in the middle row illustrate the results when PA=10 degrees (i.e., the pantoscopic angle set as the standard value) and the graphs (I) to (L) in the lower row illustrate the results when PA=20 degrees.
In the respective rows, the graphs (A), (E) and (I) show the vertical cross-section of the lens, the graphs (B), (F) and (J) show the surface power of the outer surface on the main fixation line, the graphs (C), (G) and (K) show the surface power of the inner surface on the main fixation line, and the graphs (D), (H) and (L) show the refractive performance of the lens when the object is seen through the lens at positions on the main fixation line (hereinafter referred to as transmission refractive power). In progressive power lenses: the distance portion is set for viewing an object long distanced from the eyes (e.g., distanced therefrom by 10 m to infinity); the near portion is set for viewing an object short distanced from the eyes (e.g., distanced therefrom by 40 cm to 25 cm); and the intermediate corridor is set such that its focal distance is gradually changed as the intermediate corridor extends from the distance portion to the near portion. With these settings, the transmission refractive power needs to be evaluated. In each graph, the vertical axis represents Y coordinates vertical from the prism reference point. The horizontal axis represents Z coordinates in the graphs for showing the vertical cross-sections while representing the refractive power in diopter in the graphs for showing the refractive powers.
The lens vertical cross-sectional views (A), (E) and (I) of FIG. 3 illustrate the cross-sectional shapes and actual postures of the spectacle lens 1 when respectively angled at the pantoscopic angles PA of 0, 10 and 20 degrees.
The suffixes m and s respectively represent relevance to a direction along the main fixation line 7 and relevance to a direction orthogonal to the main fixation line 7. The suffixes 1 and 2 respectively represent relevance to the outer surface and relevance to the inner surface. When N represents a refractive index of the lens material, C represents a curvature of the surface (unit: 1/m) and D represents a refractive power (unit: diopter), the following expressions are possible:    Cm1: curvature of a cross-section of the outer surface taken along the main fixation line;    Cs1: curvature of a cross-section of the outer surface taken orthogonally to the main fixation line;    Cm2: curvature of a cross-section of the inner surface taken along the main fixation line;    Cs2: curvature of a cross-section of the inner surface taken orthogonally to the main fixation line;    Dm1: surface power of the cross-section of the outer surface taken along the main fixation line=(N−1)·Cm1;    Ds1: surface power of the cross-section of the outer surface taken orthogonally to the main fixation line=(N−1)·Cs1;    Dm2: surface power of the cross-section of the inner surface taken along the main fixation line=(1−N)·Cm2; and    Ds2: surface power of the cross-section of the inner surface taken orthogonally to the main fixation line=(1−N)·Cs2.
Further, by similarly using the suffixes m and s for respectively representing the relevance to the direction along the main fixation line and the relevance to the direction orthogonal to the main fixation line, the transmission refractive power on the main fixation line can also be expressed as follows:    Pm: a transmission refractive power at the cross-section of the lens taken along the main fixation line; and    Ps: a transmission refractive power at the cross-section of the lens taken orthogonally to the main fixation line.
According to Conventional embodiment 1, no custom designing in view of the pantoscopic angle PA is conducted. Thus, even when the pantoscopic angle PA is changed in accordance with the fitting conditions for manufacturing a user's spectacles, the lens designed for the standard pantoscopic angle PA of 10 degrees is merely put in a changed posture with the same designing maintained. Hence, no matter whether the pantoscopic angle PA is 0, 10 or 20 degrees, the refractive powers of the outer surface and the inner surface remain unchanged. Specifically, the spectacle lens of Conventional embodiment 1 has its progressive surface on the inner surface, so that the outer surface is spherical. Since the base curve is 3.0 diopter (hereinafter, the unit diopter will be abbreviated as (D)), Dm=Ds=3.0 is satisfied constantly in the entire region on the main fixation line. In FIG. 3, Dm represented by a solid line is overlapped with Ds represented by a dotted line to form a single line.
On the other hand, in the progressive surface of the inner surface, the intermediate corridor extends from y=3 mm to y=−11 mm, within which the refractive power is increased from −7 (D) to −5 (D) by the addition power of 2 (D). In the distance portion located above y=3 mm, the surface power Dm2 of the cross-section taken along the main fixation line (represented by a solid line) is gradually shifted toward the positive side as it extends upwardly while the surface power Ds2 of the cross-section taken orthogonally to the main fixation line remains substantially constant. In the intermediate corridor, Dm and Ds are substantially equal. In the near portion, Ds remains substantially constant as it extends downwardly while Dm is slightly shifted toward the negative side. The difference between Dm and Ds means that the lens has a surface astigmatic power. Although the prescription of this lens does not include a prescription for astigmatism, the lens is designed to include a surface astigmatic power in order to correct an aberration generated when the line of sight obliquely passes through the lens at the periphery. This designing technique is utilized in designing progressive power lens in recent years (hereinafter called as off-axis aberration aspherical correction). Advantages of this designing technique are understandable from the graph showing the transmission refractive power at PA of 10 degrees. Specifically, when PA=10 degrees, the transmission refractive power Pm of the cross-section taken along the main fixation line equals to the transmission refractive power Ps of the cross-section taken orthogonally to the transmission refractive power Pm, so that Pm represented by a solid line is overlapped with Ps represented by a dotted line in the graph. In other words, the targeted refraction at the distance portion, intermediate corridor and near portion is obtained.
In contrast, when the pantoscopic angle PA is 0 degree, an astigmatic aberration of approximately 0.3 (D) is generated in the distance portion, and when the pantoscopic angle PA is 20 degrees, an astigmatic aberration of approximately 0.6 (D) is generated in the distance portion (see, the graphs (D) and (L) of FIG. 3).
Considering that a typical lens is prescribed in units of 0.25 (D), the above aberrations are considerably large optical errors. Not only the astigmatic aberrations are present, but also an average refractive power defined as an average between Dm and Ds is deviated from the targeted constant value.
Next, Conventional embodiment 2 will be described below. A progressive power lens of which distance portion is prescribed for hyperopia use will be described as Conventional embodiment 2. The progressive power lens of Conventional embodiment 2 has: a spherical power SPH of 4.00 diopter; a cylindrical power CYL of 0.00 diopter; a cylinder axis AX of 0 degree; an addition power ADD of 2.00 diopter; a corridor length FL of 14 mm; a refractive index N of 1.60; a pantoscopic angle PA of 10 degrees; and a base curve BC of 6.00 diopter.
FIG. 4 is for explaining the optical characteristics on the main fixation line when the progressive power lens of Conventional embodiment 2 is worn at different pantoscopic angles. As in FIG. 3, the graphs (A) to (D) in the upper row illustrate the results when the pantoscopic angle PA=0 degree, the graphs (E) to (H) in the middle row illustrate the results when PA=10 degrees and the graphs (I) to (L) in the lower row illustrate the results when PA=20 degrees. In the respective rows, the graphs (A), (E) and (I) show the vertical cross-section of the lens, the graphs (B), (F) and (J) show the surface power of the outer surface on the main fixation line, the graphs (C), (G) and (K) show the surface power of the inner surface on the main fixation line, and the graphs (D), (H) and (L) show the refractive performance of the lens when the object is seen through the lens at positions on the main fixation line.
Conventional embodiment 2 is different from Conventional embodiment 1 in that the progressive surface is on the outer surface. The outer surface is the progressive surface, and the intermediate corridor extends from y=3 mm to y=−11 mm. Within this range, the surface powers Dm and Ds on the main fixation line are increased from 6 (D) to 8 (D) by the addition power of approximately 2 (D). In the distance portion located on and above y=3 mm, Ds remains substantially constant while Dm is gradually decreased as it extends upward. On the other hand, in the near portion, Ds remains substantially constant while Dm is gradually decreased as it extends downward. The reason therefor is that the designing technique of the off-axis aberration aspherical correction is employed as in Conventional embodiment 1. As a consequence, it is understandable from the graph (H) of FIG. 4 that the targeted optical characteristics are obtained at PA of 10 degrees. However, when PA is 0 degree, large astigmatic aberrations of approximately 0.4 (D) and 0.7 (D) are generated respectively in the distance portion and the near portion, and when PA is 20 degrees, large astigmatic aberrations of approximately 0.8 (D) and 0.3 (D) are generated respectively in the distance portion and the near portion.
As described above, hitherto-manufactured progressive power lenses are designed with the pantoscopic angle being fixed at the standard value. Thus, when the pantoscopic angle cannot be adjusted to be the standard value due to restrictions on structures and materials originated from designs of spectacle frames, the targeted optical performance of the lens will not be obtained and, even worse, the optical characteristics will be greatly deteriorated. Further, it is time- and labor-consuming for retailers to adjust the pantoscopic angle every time.
On the other hand, under an idea of customization designing of progressive power lenses, Document 1 and the like already indicate a concept of conducting optical corrections depending on elements such as the pantoscopic angle, but suggest nothing beyond a rough outline as to the method for realizing the concept. None of Documents 1 to 3 discloses a specific structure of the progressive power lens adaptable to various conditions such as lens prescription, and thus the concept remains impracticable.